Glucose
(Glc), a monosaccharide, is one of the most
important carbohydrates. The cell uses it as a
source of energy and metabolic intermediate.
Glucose is one of the main products of
photosynthesis and starts cellular respiration.
The natural form (D-glucose) is also referred to
as dextrose, especially in the food
industry. This article deals with the D-form of
glucose (see Isomers-section
below)

Glucose

Chemical
name

6-(hydroxymethyl)oxane-2,3,4,5-tetrol

Synonym
for D-glucose

dextrose

Varieties
of D-glucose

α-D-glucose;
β-D-glucose

Abbreviations

Glc

Chemical
formula

C6H12O6

Molecular
mass

180.16
g mol−1

Melting
point

α-D-glucose:
146°C
β-D-glucose: 150°C

Density

?
g cm-3

CAS
number

50-99-7
(D-glucose)

CAS
number

921-60-8
(L-glucose)

SMILES

C(C1C(C(C(C(O1)O)O)O)O)O

Structure

Glucose
contains six carbon atoms and an aldehyde group
and is therefore referred to as an aldohexose.
The glucose molecule can exist in an open-chain
(acyclic) and ring (cyclic) form, the latter
being the result of an intramolecular reaction
between the aldehyde C atom and the C-5 hydroxyl
group to form an intramolecular hemiacetal. In
water solution both forms are in equilibrium,
and at pH 7 the cyclic one is the predominant.
As the ring contains 5 carbon and one oxygen
atoms, which resembles the structure of pyran,
the cyclic form of glucose is also referred to
as glucopyranose. In this ring, each carbon is
linked to an hydroxyl side group with the
exception of the fifth atom, which links to a
sixth carbon atom outside the ring, forming a CH2OH
group.

Space-filling
model of glucose molecule

Isomers

Glucose
has 4 optic centers which means that in theory
glucose can have 15 optical stereoisomers. Only
7 of these are found in living organisms, and of
these galactose (Gal) and mannose (Man) are the
most important. These eight isomers (including
glucose itself) are all diastereoisomers in
relation to each other and all belong to the
D-series.

An
additional asymmetric center at C-1 (called the
anomeric carbon atom) is created when
glucose cyclizes and two ring structures, called
anomers, can be formed — α-glucose and
β-glucose. They differ structurally in the
orientation of the hydroxyl group linked to C-1
in the ring. When D-glucose is drawn as a
Haworth projection, the designation α
means that the hydroxyl group attached to C-1 is
below the plane of the ring, β means
it is above. The α and β forms
interconvert over a timescale of hours in
aqueous solution, to a final stable ratio of
α:β 36:64, in a process called mutarotation.

PRODUCTION

Natural

In
animals and fungi, glucose is the result of
the breakdown of glycogen, a process known
as glycogenolysis. In plants - the breakdown
substrate is starch.

In
animals, glucose is synthesized in the liver
and kidneys from non-carbohydrate
intermediates, such as pyruvate and
glycerol, by a process known as
gluconeogenesis.

Glucose
shifting from Fischer projection

to
Haworth projection

Commercial

Glucose
is produced commercially via the enzymatic
hydrolysis of starch. Many crops can be used as
the source of starch Maize, rice,
wheat,
potato, cassava, arrowroot, and sago are all
used in various parts of the world. In the United
States, cornstarch (from maize) is used
almost exclusively.

This
enzymatic process has two stages. Over the
course of 1-2 hours near 100 °C, these enzymes
hydrolyze starch into smaller carbohydrates
containing on average 5-10 glucose units each.
Some variations on this process briefly heat the
starch mixture to 130 °C or hotter one or more
times. This heat treatment improves the
solubility of starch in water, but deactivates
the enzyme, and fresh enzyme must be added to
the mixture after each heating.

In
the second step, known as saccharification,
the partially hydrolyzed starch is completely
hydrolyzed to glucose using the glucoamylase
enzyme from the fungus Aspergillus niger.
Typical reaction conditions are pH 4.0–4.5, 60
°C, and a carbohydrate concentration of
30–35% by weight. Under these conditions,
starch can be converted to glucose at 96% yield
after 1–4 days. Still higher yields can be
obtained using more dilute solutions, but this
approach requires larger reactors and processing
a greater volume of water, and is not generally
economical. The resulting glucose solution is
then purified by filtration and concentrated in
a multiple-effect evaporator. Solid D-glucose is
then produced by repeated crystallizations.

Function

We
can speculate on the reasons why glucose, and
not another monosaccharide such as fructose
(Fru) , is so widely used. Glucose can form from
formaldehyde under abiotic conditions, so it may
well have been available to primitive
biochemical systems. Probably more important to
advanced life is the low tendency of glucose, by
comparison to other hexose sugars, to
non-specifically react with the amino groups of
proteins. This reaction (glycation) reduces or
destroys the function of many enzymes. The low
rate of glycation is due to glucose's preference
for the less reactive cyclic isomer.
Nevertheless, many of the long-term
complications of diabetes (e.g., blindness,
kidney failure, and peripheral neuropathy) are
probably due to the glycation of proteins or
lipids. Glycosylation is another important type
of reaction undergone by glucose.

As
an energy source

Glucose
is a ubiquitous fuel in biology.
Carbohydrates are the human body's key source of
energy, providing 4 kilocalories (17 kilojoules)
of food energy per gram. Breakdown of
carbohydrates (e.g. starch) yields mono- and
disaccharides, most of which is glucose. Through
glycolysis and later in the reactions of the
Citric acid cycle (TCAC), glucose is oxidized to
eventually form CO2 and water,
yielding energy, mostly in the form of ATP. The
insulin reaction, and other mechanisms, regulate
the concentration of glucose in the blood. A
high fasting blood sugar level is an indication
of prediabetic and diabetic conditions.

As
a precursor

Glucose
is critical in the production of proteins and in
lipid metabolism. Also, in plants and most
animals, it is a precursor for vitamin
C (ascorbic acid) production.

Glucose
is used as a precursor for the synthesis of
several important substances. Starch, cellulose,
and glycogen ("animal starch") are
common glucose polymers (polysaccharides).
Lactose, the predominant sugar in milk, is a
glucose-galactose disaccharide. In sucrose,
another important disaccharide, glucose is
joined to fructose.

Sources
and absorption

All
major dietary carbohydrates contain glucose,
either as their only building block, as in
starch and glycogen, or together with another
monosaccharide, as in sucrose and lactose. In
the lumen of the duodenum and small intestine
the oligo- and polysaccharides are broken down
to monosaccharides by the pancreatic and
intestinal glycosidases. Glucose is then
transported across the apical membrane of the
enterocytes by SLC5A1 and later across their
basal membrane by SLC2A2 (ref). Some of glucose
goes directly to fuel brain cells and
erythrocytes, while the rest makes its way to
the liver and muscles, where it is stored as
glycogen, and to fat cells, where it is stored
as fat. Glycogen is the body's auxiliary energy
source, tapped and converted back into glucose
when there is needs for energy.